1. Field of the Invention
The present invention generally relates to systems and methods for inspecting a specimen. Certain embodiments relate to systems and methods for inspecting a specimen that includes a substantially rough uppermost layer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various times during a semiconductor manufacturing process to detect defects on wafers. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Many different types of inspection systems have been developed for the inspection of wafers. One example of an inspection system that is configured to inspect relatively smooth surfaces such as those of bare silicon wafers uses oblique illumination with a polarization combination called “P-U,” which indicates that the incident light is P-polarized (i.e., polarized in the plane of incidence) and the scattered light that is detected is unpolarized (i.e., light having all polarizations is collected and detected). The sensitivity of such a system is greatest for inspection of substantially smooth surfaces. However, relatively rough surfaces scatter a substantial amount of light in the P-U configuration. In this manner, scattering from relatively small defects can be much lower than the surface scattering. As such, the signal-to-noise ratio of the inspection data may not be high enough to allow accurate detection of relatively small defects.
In another example of a wafer inspection system, a “double-dark field” configuration can be used for inspecting relatively rough surfaces for contamination and other defects. In particular, using S-polarized (i.e., polarized perpendicular to the plane of incidence) obliquely incident light results in a dark fringe at the surface, which produces substantially little light scattered from the surface itself. Such illumination used with an analyzer oriented perpendicular to the plane of scatter and an aperture limited to “side-angle collection” can reduce the contribution of unwanted surface scattering to the background noise by several orders of magnitude. Large particles and defects located on the surface of the wafer can be detected relatively easily using this configuration since they do not experience the dark fringe effect and therefore perturb (or scatter) the incident electric field efficiently compared to the surface.
Side-angle collection typically involves limiting the collected scattered light to azimuthal angles reasonably close to +/−90 degrees with respect to the plane of incidence. For example, in the SP1-TBI system that is commercially available from KLA-Tencor Corporation, San Jose, Calif., there are two configuration for side-angle collection: one that collects light within 10 degrees of +/−90 degrees (i.e., a 20 degree azimuthal width on each side of the plane of incidence), and one that collects light within 20 degrees of +/−90 degrees (i.e., 40 degrees of azimuthal width on each side of the plane of incidence).
The S-S polarization combination with side-angle collection configuration works well for particles having a size greater than approximately one-half the wavelength of the incident light. Such defect detection capability is achievable due, at least in part, to the fact that the S-S side-angle configuration is substantially effective at reducing the scattering from the surface. Unfortunately, this configuration is also substantially effective at reducing the scattering from small defects, which are generally defined herein as defects having a size that is smaller than one-half the wavelength of the incident light. However, once the defect size increases to approximately one-half the wavelength of the light or greater, a typical defect begins to scatter significantly into the side-angle collection space. Since the surface scattering is suppressed, this configuration provides a significant signal-to-noise advantage for the inspection system for the detection of relatively large defects on relatively rough surfaces.
Between about 1993 and about 1998, semiconductor processes using material that have rough surfaces were subject to failure caused by defects having a size of approximately 200 nm and larger. Therefore, the desired defect detection capability could be achieved by using an illumination wavelength of 488 nm, which Surfscan instruments that are commercially available from KLA-Tencor, used at the time. But Moore's law marches on, and today customers are expressing the need to detect defects having a size of 150 nm, 100 nm, or even smaller, on wafers having even relatively rough surfaces. Therefore, even an ultraviolet (UV) wavelength of, say, 355 nm combined with the S-S side-angle technique is not sufficient for detecting defects of such sizes on wafers having a relatively rough upper surface.
Accordingly, it may be advantageous to develop systems and methods for inspecting a specimen, particularly a specimen having a relatively rough uppermost layer, that are capable of detecting defects having sizes that are less than about half of the incident wavelength used for inspection with relatively high accuracy.